This invention relates to improvements in electromechanical actuators, especially for use in a hydraulic braking system for a vehicle.
In a traditional hydraulic braking system, brake pressure is created by the driver pressing on a foot pedal, which moves a push rod that in turn displaces a diaphragm in a vacuum boosted brake servo. The servo amplifies the pressure applied by the driver, and generates an increased hydraulic pressure in a hydraulic fluid connected to a brake caliper or drum.
It is known also to provide a braking system in which the pressure applied by the brake is controlled by an electromechanical actuator. This allows the brakes to be applied using force from the actuator rather than purely relying on the driver pressing the pedal. This arrangement also allows the brakes to be applied independent of the driver, for instance during an emergency or as part of a vehicle stability control system or a hill hold function.
A suitable actuator should have a fast response time and be capable of generating a relatively large pressure increase, and most conveniently the actuator is capable of driving a piston back and forth within a cylinder. It will be appreciated that an actuator capable of such performance can be used in a wide range of applications, and is not just limited to use in a hydraulic braking system.
In one arrangement, disclosed in U.S. Pat. No. 4,909,577, an electromagnetic actuator comprises a motor having a stator and a rotor. The rotor is hollow and is provided on an inner surface with a helical groove that forms part of a ball screw linear drive located inside the rotor. The drive comprises an elongate screw having an external helical groove that complements that of the rotor. The two grooves are filled with metal balls and the balls act on the screw to create a linear movement of the screw relative to the stator. The linear movement of the screw is used to control the hydraulic pressure in a hydraulic braking circuit of a vehicle. The rotor is supported by bearings at both ends.
In an alternative arrangement disclosed in the applicant's earlier patent EP0317182 a stepper motor has a screw that is fixed to the protruding end of the rotor of a motor so that as the rotor rotates the screw also rotates. The screw protrudes into a bore formed in an end face of a piston that is in turn inserted in the bore of a cylinder filled with hydraulic fluid. The piston is prevented from rotating in the cylinder, and as the rotor rotates, the piston is moved along the cylinder. This alters the pressure of the fluid in the hydraulic brake circuit connected to the cylinder.